Multidisciplinary study (geochemical and foraminiferal assemblages) to characterize and assess anthropic activities at the Araçá Bay, São Sebastião Channel, Brazil

Abstract

The goal of this study was to assess possible human-induced changes in the environment due to the discharge of domestic sewage into the Araçá Bay, São Sebastião Channel, São Sebastião, Brazil. In order to achieve this goal, in April 2006, we conducted grain size and geochemical (Toc, N, S, and P) analysis and determined the assemblage composition and distribution of live benthic foraminifera from shallow marine sediments collected around diffusers of the domestic sewage submarine outfall at the Araçá Bay. Results showed that the area studied is a lithoclastic shallow environment with predominance of sandy sediments. However, in the northeastern area of the sampling grid, it was observed high concentrations of fine sediments (silt plus clay), with significant Toc (>; 1%), N (>; 0.2%) and S (0.3%) contents. The organic enrichment is inferred based on the phosphorus and sulphur contents, as well as on the oxidation-reduction potential, which was inferred using the Eh values and C/S ratios. In relation to the benthic foraminiferal assemblages, low richness was observed and the association is mainly composed of infaunal and detritivore species. There is a high abundance of Epifaunal species, which are bioindicators of well-oxygenated environments, in stations with no impact of the sewage discharged by submarine outfall. The occurrence of medium and small-sized foraminifera shells is qualitatively associated with low oxygen concentrations at the sediment-water interface. The dominance of a few species, especially Ammonia tepida, may indicate environmental stress.